The concept of the oceanic lithosphere as a cooling thermal boundary layer on top of a viscous upper mantle accounts for the increase in depth with age of the ocean floor. Analyses of the exact nature of this relation provide constraints on the rheology and structure of this layer. In past studies, problems with uncertainties of age and in sediment thickness have limited the value of this relation on older ocean floor. In this study we evaluate different tectonic histories for the ''quiet zones'' and construct a revised isochron chart for the North Pacific. We check and modify the current sediment thickness charts using new deep-sea drilling data and revise ideas concerning reflectors in the southwestern North Pacific. Averaging the SYNBAPS digital data base into half-by-half-degree elements and correcting for sediment load, we plot basement depth versus age for the entire North Pacific. Because of the skewness in the data associated with seamounts, we present the data as countours about the mode and find that two-thirds of the observations lie within 300 m of this value. On crust between 0 and 80 Ma, the ocean floor subsides with the square root of age. On ocean floor older than 100 Ma the depths in general flatten with age with areally correlateable swells and valleys. The Hawaiian and Line Island swells correlate positively, and the Mariana basin valley negatively, with geoid height. The flattening in the depths indicates that the thermal boundary layer has reached equilibrium conditions under older ocean floor. Further, the correlation of the swells and valleys with geoid height present evidence for organized convection in the upper mantle beneath older lithosphere in the North Pacific. ¿ American Geophysical Union 1988